Testing the hypothesis that heart rate, via visceral afferent feedback to the central nervous system, directly influences sensorimotor behavior, the following experiments are planned: (1) The relationship between fore-period effects on reaction time and anticipatory cardiac deceleration will be determined for equal-N and equal-probability schedules of foreperiods; (2) The effect of atropine, which blocks anticipatory cardiac decelerations, on reaction time at varying foreperiods will be determined; (3) The effect on reaction time of time within the cardiac cycle at which stimulation occurs will be determined for two points in the cardiac cycle - the P-wave and the descending limb of the T-wave; (4) The effects of sinusoidal perfusion of the carotid sinus, at varying frequencies, on evoked potentials in nucleus cuneatus will be determined in acute cat preparations; (5) The role of respiratory changes and metabolic demands in producing anticipatory cardiac decelerations will be evaluated by breath-to-breath measurements of oxygen consumption. Based on previous work in this laboratory showing that heart rate is differentially affected by variations in the time of stimulation within the cardiac cycle, "pacemaker response curves" will be determined in intact man, and preliminary explorations will be made of the potential clinical applicability of such curves.